Capacitance voltage divider



Aug. 20, 1963 M. .1. FRIED CAPACITANCE VOLTAGE DIVIDER Filed Nov. 6, 1961 OUT PUT INVENTOR. MORTON J. FRIED A T TORNE YS United Statesr The present invention relates to` electrical voltage dividers 4and more particularlyto` capacitive voltage dividers utilized in conjunction with high voltages.

Diagnostic investigation. of electricalsignals requires the use of numerous sensing instruments `and recording devices. ,These-electrical instruments and devices :are for the most part designed to accept and respond to electrical signals having voltage potentials of a few hundred volts or less.` Electrical signals having higher voltage potentials result in either damage to the equipment :or inaccurate information due to instrument operation outside lofits designed limits. j n

The problem presented` when an electrical signal of many hundreds or thousands of volts contains information which it is desirable to extractor record, is not solved by merely designing equipment with greater-operating arent limits. The operating limits oftmost diagnostic equipment is set, not' arbitrarily, but by the components which make up the instrument. Vacuum tubes, cathode ray tubes, transistors, photo tubes, resistors, capacitors, etc., all' have limits as to the voltages at which they can operate and' consequently limit the instruments of which they are a part.` Thus diagnostics of signals having high volt-ages require that the signal Ibe operated on to reduce-its: voltage magnitudehefore it is presented to the various instruments and devices Ifor obtaining desired data.

Although voltage divisionV by means of resistive elements (resistors) isthe common method lor reducing voltage magnitudes, this method has a number of serious limitations when electrical signals which have magnitudes in the kilovolt region and vfrequencies in the megacycle region are under investigation. The most serious limitation of resistors isY their inability` to produce the necessary voltage division (1 0-00 to l or'more) Vof `a vhigh .frequency signal without greatly distorting the lshape of the signal duetoV the largeinductance inherent in resistive devices havingthe ability to handle yhighvoltages and large power dissipations. Since it is desirable to .greatly reduce the magnitude of thesignal and at fthe same time maintain the signal shape as nearly iasposs-ible, voltage division by capacitive elements,.rather thaneresistive element-s, is preferable. v

Whena'signal voltage is lapplied across a pair of capacitors connected in series, lthe vvoltage 4across each capacitor isl inversely proportional .to the ratio vof its capacitance to the combined capacitance of the two capacitors, regardless ofthe signal frequency or shape. Since the intrinsic inductanceof some types of capacitors, even those rated as high as 35 kv. (kilovolts) is very small,

. large voltage reductionV with minimum ydistortion is achieved. v

'When theV electrical signal under study is a sinusoidal signal with low harmonic content, capacitance dividers as known in the art perform the desired voltage, divisi-on with la minimum of undesirable sideeifects. When, however, the signal* to be studied and analyzedV is a fast rise time pulse or other sign-a1 having high harmonic content', capacit-ance dividers known in the Iartexperience an `oscillating effect, knownas ringing, which all ibut obliterates they signal being operated on. Another characteristic of capacitance dividersused in conjunction- With highenergy (high voltage) pulse signals, is the danger presented when a system failure causes large quantities of energy from the signal source to be @absorbed by the capacitors of the divider network. A capacitance divider characteristically includes a vacuum capacitor Ihaving a high voltage rating and which serves to drop the high voltage.v Whenthis. v

picked up `and enter the system` in thek form of noisei Under certain conditions this source of noise can greatly` reduce the accuracy of the information obtainedf Accordingly, it is an object of the present inventionto provide a capacit-ance voltage divider for decreasing the magnitude of` high voltage electrical signals, wherein p signals having high harmonic content will not induce the system tering. j y

Another object of the present invention is to provide a capacitance voltage Idivider which is packaged in such a mannerras to provide safety in case of component failure and eliminate undesirable side effects from signals having l high harmonic content.y

It is a further object lof the present invention to provide a capacitance; voltage divider capablejof reducing awfalst rise time electrical signal voltage -by la lange factor, etg;

1GO-0, Without atten-dant ringing. Another object vof this invention is to provide a high voltage capacitancedivider which cannot introduce noise by way of radiated'signals int-o an attendant system.y

Further and more specilic objects and advantagesof the invention lare made vapparent in the following; speci oation wherein a preferred form of the invention is described by reerence'to the accompanying drawing.

FIG. l is an isometric View of the capacitor divider of the present invention;

FIG. 2 is an electrical` circuit diagram of the capacitor y divider of thepresent` invention; and f FIG. 3 is a verticalsection View of FIG.. l..

Referring now to FIG. l, a rigid cylindrical, metal housing 11 contains `the electrical components of the capacitance divider-12' of the present invention. A high voltage input terminal 13k, coaxial output connector 14 and commonground connector 16 provideme'a-ns for gaining electrical access to the components contained within housing l1. A cylinder of insulating material 17 extends beyond the upper edge 18y of housing 11 approximately the same ldistance as high voltage input terminal 15. The high voltage `input terminal is thereby completely surrounded by insulating material to prevent the occurrence of arcing between terminal 13 and housing 1d,` or other metallic objects in close proximity.y A plurality of port holes 19 in the lower portion of housing llly proyide'escape means for gases which might otherwise build up high enough pressures to causean explosion when anjelectrical failure took place;l Similar port holes 21 are provided in the upper end closure 2,2 for the same reason. Itv will become obvious from `the yfollowing description thatrthe cylindrical geometry of housing 1:1' is nota necessary requireme-nt of the invention; ibut rather one that lendsitself wellwith the othercomponents. A

v Referring now to FIG.` 2, Iahighvoltage.` signal tohe reduced inmagnitude before lbeingL introducedi intofan analyzing system or the like is placed across input termi-` nals 13,y ld. The high voltage signal thus occursiacross the series connection. of capacitors 23 and 24 to which' terminals 113, 16 `are connected, respectively. The output, from the capacitance voltage divider 1-1 appears at ter-rni-, nals 14, 16 which are connected at'either end of capacitor 24 land thereforemakes accessible the voltagewhich appears across capacitorld. The apparatus (not shown) f estaban.,-

Fmentgattached thereto would result.

to receive vthe signal from output terminal-s 14, lo should have 'La high input impedance, or an intermediary circuit Y for establishing a high impedance load for divider 11 should be furnished. A resistor `26 in series 'with output terminal 14 andcapacitor v23,and disposed therebetween, while'not necessary is most helpful in reducing ringing when `high frequency signals are being acted upon. Ca-

paci'tor 23 is of the high voltage, low capacitance variety.V

Such `capat'sito-rs are characteristically vacuum capacitors having capacitance ratings in the micro-micro -farad region and voltage ratings of up to 35 kilovolts.y Capacitor 24,

Y `r on -the other hand, is of the low inductance variety, such as mica or ceramic capacit-ors, and need have a voltage rating of only 100 volts or so. The ratio between the capacitanc'evalue of capacitor Z4 rand the capacitanceV value of capacitor A2.1i will be the ratio by which the input voltage is reduced'in'rea'ching the output. Thus'if capacitor 23 is,rfo`r example, a 10 micromicrofarad capacitor and capacitor-24 isa 10,000 micromicrofarad capacitor, the` magnitude ofthe voltage at output terminals 14, 16

willffbecne-thousandthof the magnitude of the voltage' Apresentedat input terminals 13, `16.

rlhe particular ratio chosen to exist between capacitors 23"'and '24 will, fof course; depend upon numerous factors which include the magnitude of the 'input signals being n dealt with, the voltage ratingl of the apparatus receiving fthe outputfsignah the capacitor voltage and current ratl ings, etc. yWhen the input signal is inthe l to` 35 kilovolt v region, avoltage division of one thousand, as mentioned above, will result in outputv voltages between l0v and 35 volts 'and is therefore a realistic and practical ratio to n Because'capacitor 23 may be called upon to drop voltage'sof up to 35 kilovolts land more, and the source pro` ducing suchv signals often is van extremely high energy source, the-possibility of other components being exposed to voltages, and absorbing energies, far in excess of their ratings, must-be considered. A failure .in capacitor 23 'may well llead to capacitor 24 having v35 kilovolts dropped across it |when its rating sets its upper limit at `a few hunf .,d-red volts. f The same is' possible as to resistor 26. A

v end 4d of fixture '316, which resides in chamber 32, are f failure-in theenergy source providing the,- signal at they input terminals may result i-n many joulesof energy flowing into the capacitance divider network and exceeding 'even the ratings of capacitor 23. 'It thus becomes imv lportant to provide for such possible malfunctions and y.failures and thereby minimize their effect. The primary concern, of course, is to the personnel who are yin the immediate vicinity of the capacitance divider yat the time suchfailure occurs.` Because the energies involved are suflicient to vaporize components `at a rate which results niny explosion, the network must be properly, packaged.

For this purpose, metal housing 11 with port holes 19 and 21' is provided (FIGS. 1 iand 3). `It then becomes fimportantto protect the equipment connected to output terminals1j4, 116)4 If the input signal should,- due to a failure of some kind, appear at its original magnitude vacross the output terminals, severe damage tothe equipy To prevent this a fuse 27 is positioned between lresistorZ and output terminal 14 so as to Iform an open circuittherebetween when the current rating of the fuse is exceeded-.1 It is altogether possible,however, with the large voltages which could be t VVVinvolved, thatpa simple fuse may not supply Ythe safety required, due to possible arcing between its end elements M even after it has blown `Thus an added safety feature is Nprovidedzin the yform of an `arc gap 28. 'When the voltage at the junction point 29, between capacitors 2.3

VandQZi, greatly exceeds the value of the voltage designed kto' exist at this-point, `an are will beoreated between the points of arc gap '2'8 and thereby form a complete circuit to ground. In this manner, the iiow'of energy is diverted from the output to ground where it can be safely dissipated. l

Referring now to FIG. 3, metal housing 11 has an upper chamber 31 anda lower chamber 32 separated by screws 34. Residing in a central bore in member `33 is a large tarea, loW inductance conductor 36. Conductor 36 has a ange 38 at one end tandis Asecured to member 33 as by screws 7 whichpass through the flange and anchor in the support member. Aiiixed to the other end of conductor 36,l which resides in chamber 31, is a capacitor terminal receivingmernber 39 which acts as a mount for' capacitor yZ3 and also forms la large surface, low inductance, yelectrical connection between capacitor 23 and conductor 36. Thus terminal 56 of capacitor 23 need not contact conductor 36 directly for proper electrical communication to exist. Electrically connected to capacitor 24 rand resistor-26. A single, large area solder connection securing botihcapacitor 2.4 and resistor 26 to conductor 36 ispreferred to minimize theinductance of the system.

Although the physical connection between electronic vcomponents is generally unimportant to the overall operation of ay given circuit, one of the critical featuresl of the present invention is the length of the conductors which connect the various components. Ithas been found that by maintaining conductors `42 and 4S to a minimum length the amount of ringing which occurs in response to a signal havinghigh harmonic content is greatly reduced. It has been found that by limiting leads 42 and 48 toa length not greater than Az of `an inchr the maximum amount ofreduction in 'ringing from this vsource is f achieved. Maintaining lead 42 to less than l Vinch proi conductor i7 which electrically connects with the `housing '11 rand thereby with ground vtermina-1116. Thus the grounded `side of capacitor 24 may be conveniently connected byv conductor 48 to' that portion oftenminal 14v which is in contact with housing 11. Inner conductor 46 and outer conductor 47 of terminal 14 are' maintained velectrically separate as by insulator 49.

"Ehe extreme'lower end of housing 111 is sealed by rigid conducting structure member 5l. which is Arrnly secured in place as by a plurality of screws 52. A threaded hole 510 in the center of member 51 provides a convenient means for mounting the invention when such is desired. vAll of the circuit components, except yfor capacitor 23, lare maintained within chamber'32,` which is completely enclosed. In the event of an electrical failure which results in vaporization of components, the components will be within chamberSZ protecting personnel l in the area. A possible explosion is prevented by allow- K port holes l19. 5

' Arc gap 28, described in connection With `FIG. 2,' formed by a screw 53 which `is secured in lend ldisc 51 and extends to within close proximity of'fxture 36. Gap

28 can, however, be fonmed by any conductive memberl secured at one end to ground and placedk in imity toy conductor36. l

Capacitor 23 which is capable of having voltages in the lcilovolt region placed across it has an evacuated yglass close proxenclosure 154 which houses the capacitive forming Aele-V ments, rand has a pair of connection terminals 56 which v' communicate electrically 'withthe' interior of the enclosure `54. As pointed out above, one terminal S6 is `supported in receivingmember 39 to Iform electrical con` towhich member 157 is secured and a standard terminal' pole 59, which is aiiixed todisc 58. Pole 59 passes through upper end cover 22 and issecured thereto las by looking nut 61. f l

The upper end 18. of Vhousing 11v is sealed by means of endl closure member 22 which is integrally connected to an upwardly extending cylinder `17 for surrounding terminal 13, and .a downwardly extending cylinder 62 for surrounding upper capacitor terminal 56. The single cap member |63 lfonmed lby cylinders I17 and 62' and top closure member 22 is constructed .from non-conductive materialhaving good insulating properties. The function of upwardly extending cylinder 17 has been explained above. Closure member 22 vserves to form completely/enclosed chamber 31 to safeguard against possible explosion or similar iailure of capacitor 23. Holes 21 in member 22 allow gases to escape from chamber 31 when a yfailure causes them to build up pressure therein. Member 22 also serves as a mounting structure for terminal 13 and capacitorterminal receiving member 57.

Cylinder 62. which extends downwardly into housing 11 surrounds upper capacitor terminal `56 which is the high voltage terminal. Voltages of up to 35 kilovolts will often appear on upper terminal 56. vSince the interior of housing 11 is not evacuated, arcing from upper terminal '56 to other conductors `is la most serious problem. The problem is initially reduced by providing insulating material 464 over the interior area oicompartment 3'1 of housing 11. A material which isrnost suitable for lining the interior of housing 11 is a sheet plastic material of polyethylene `tereph-thalate, sold by the E. I. du Pont C0.. under the name Mylan Although numerous materials are suitable this material is especially attractive because of its high insulating characteristics even in rather thin sheets of material. Even with the Mylar lining, however, it has been found that varoing'will occur between upper terminal 56 (or the conductive members in contact therewith such as members 57 or 58) land the upper end of housing 11. Downwardly extending cylinder 62 provides the additional insulation around high voltage terminal 56 to prevent rarcing. The lower terminal 56 is thelow voltage terminal, not expected to be at any more than about 100 volts potential at any time, and therefore does not pose an arcing problem.

Cap member 63 can be vsecured in place in any of a variety of ways with the requirement that no metallic screws or the like protrude into the'insnlator so as to establish a conductive path which might induce arcing. The present invention shows as an embodiment in which a plurality of screws 65 passthrough threaded holes in housing 11 :and abut rather than penetrate cap 63 and thereby maintain it in position by simple compressive Lforces. i l l v Ground terminal 16 is secured to housing 411 by any appropriate means which `forms an electrical contact therewith Whether it be a rive-t or a bolt` or other such securing A capacitance voltage divider in accordance with the teachings of the present invention and having the following specification:

Size-l0 inches long by .3 inches in diameter Capacitor 23T-6 micromicrofarad vacuum capacitor with 35.. ykv. rating i Capacitor 2li-6x103 micromicrofaradr mica capacitor Resistor 26-220 ohms Load-Z0 micromicrofarad l megohm Lead from terminal 14 to load- 2 ft. RG--Z/U cable Length of leads 42 and 48%1/8 produced an output pulse (onto the screen of an oscilloscope) which had al .0l4rmicrosecond rise and fall time, in response to a 20 kvinpu-t pulse 'which had a rise and fall time of .02 microsecond. There 'was novisible ring ing of the lreproduced pulse even though it wasv one oi high harmonic content.

Although the invention has been described mainly with reference to an upper signal voltage limit off3`5 kv. and la capacitor 23 of l0l micromicrofarads, aseries connection of two vacuum capaci-tors each having twice the capacitance value that a single capacitor would be designed to have, would allow voltages of 70 kv. ormore tobe handled. Capacitor 23 is generally most. advantageously of the lowest capacitance value that such a capacitor can be obtained at. 41 micromicrofarad capacitors of this description are obtainable `from some sources. The invention is thus capable of outstanding performance with fa variety of component values as long Iasr the manner of arranging and packaging the components is in accord with the teachings herein.

What is claimed is: y

1. In a capacitance vol-tage divider the combination comprising a rigid metal cylindrical' housing, a high voltage vacuum capacitor having a high voltage terminal and a low volta-ge terminal axially mounted within said housing, lan input terminal electrically connected to the high voltage terminal of said capacitor and extending exteriorly 'of said housing, insulating material 'disposed between said housing Iand said capacitor, a low inductance capacitor mounted within said housing electrically connected at one of its sides to the low voltage terminal of said vacuum capacitor and having la capacitance value at least an order of magnitude -greater than said vacuum capacitor, an output terminal extending exteriorly of said housing and electrically connected to the low voltage terminal of said vacuum capacitor, a ground terminal aixed to said housing and in electrical contact therewith, the other side of said low inductance capacitor electrically connected to said lground'terminal, and end members secured at either end of said housing.

2. The capacitance voltage divider of claim 1 further comprising a resistive element electrically interposed between said output terminal fand the low voltage terminal of said vacuum capacitor.

' 3. In a capacitance voltage divider the combination comprising a cylindrical metal housing, a non-conductive support member having a central bore therethrough secured within said housing and dividing said housing into an upper and lower chamber, a low inductance conductive member secured within the central bore of said support member `and having an upper face in said upper chamber and la lower face in said lower chamber, a vacuum capacitor having a low voltage terminal and a high voltage terminal laxially mounted within said upper chamber, a

, first vacuum capacitor mounting mean-s secured-to the upper face of said conductive member and yforming a low inductance electrical connection Abetween the low voltage terminal of said vacuum capacitor and said conductive member, an upper cap member of non-conductive material secured to the upper end of said housing, a high voltage connection terminal extending through said upper cap, a second vacuum capacitor mounting means within said housing and secured to said high voltage connection ter- 7 minal and forming -a low inductance electrical connection between said connection terminal and the high voltage terminal of said .vacuum capacitor, a low inductance capacitor `having a capacitance value at least one order oimagnitude greater than said vacuum-capacitor mounted injsaiid 4lower chamber, a rst conductor electrically connecting one side of said low inductance capacitor to the lower face of said conductive member, an output terminal accessible from the exterior of said housing, a second conductor electrically connecting said output terminal to the lower face `of said conductive member, a third conductor electrically connecting the other side of said low inductance capacitor to said housing, a ground terminal aixed to and in electrical contact'with said housing, and a lower end cap of conductive material secured to the lower vend of said housing. 4

4. The capacitance voltage divider of claim 3 wherein said iirst and second conductors are less than one-half inch in length.

5.Y The capacitance voltage divider of claim 3 further comprising `alresistive'element electrically interposed iny said second conductor.

6. The capacitance voltage divider of claim 3 further comprising a series connected resistive element and cnrfrcnt limiting 'fuse interposed lin said second conductor,

and a gap forming conductor electrically Iconnected at one end to said housing `and in close proximity to said conductive member at its other end.

7. The capacitance voltage divider of claim 3 further comprising insulating material `disposed between said vacuum capacitor and said housing. y y

8. The capacitance voltage divider of claim 7 further comprisingia first cylindrical member of insulating material integrally connected tojsaid upper Y'cap and surrounding atv a distance said high voltage connection terminal, a

. second cylindrical member of insulating material within said housing integrally connected to said upper capi and surround said high voltage terminal of 'said vacuum capacitor.`

9. The capacitance voltage divider of claim 8 wherein said upper chamber vand said =lower chamber both have exhaust port holes therein.

References Ctedin the le of this patent UNITED STATES PATENTS 2,255,502 f Bouman M sepw, 194,1,`

`Rnderfer Aug. 2, 1955 

1. IN A CAPACITANCE VOLTAGE DIVIDER THE COMBINATION COMPRISING A RIGID METAL CYLINDRICAL HOUSING, A HIGH VOLTAGE VACUUM CAPACITOR HAVING A HIGH VOLTAGE TERMINAL AND A LOW VOLTAGE TERMINAL AXIALLY MOUNTED WITHIN SAID HOUSING, AN IMPUT TERMINAL ELECTRICALLY CONNECTED TO THE HIGH VOLTAGE TERMINAL OF SAID CAPACITOR, A LOW INDUCTANCE CAPACITOR OF SAID HOUSING, INSULATING MATERIAL DISPOSED BETWEEN SAID HOUSING AND SAID CAPACITOR, A LOW INDUCTANCE CAPACITOR MOUNTED WITHIN SAID HOUSING ELECTRICALLY CONNECTED AT ONE OF ITS SIDES TO THE LOW VOLTAGE TERMINAL OF SAID VACUUM CAPACITOR AND HAVING A CAPACITANCE VALUE AT LEAST AN ORDER OF MAGNITUDE GREATER THAN SAID VACUUM CAPACITOR, AN OUTPUT TERMINAL EXTENDING EXTERIORLY OF SAID HOUSING AND ELECTRICALLY CONNECTED TO THE LOW VOLTAGE TERMINAL OF SAID VACUUM CAPACITOR, A GROUND TERMINAL AFFIXED TO SAID HOUSING AND IN ELECTRICAL CONTACT THEREWITH, THE OTHER SIDE OF SAID LOW INDUCTANCE CAPACITOR ELECTRICALLY CONNECTED TO SAID GROUND TERMINAL, AND END MEMBERS SECURED AT EITHER END OF SAID HOUSING. 